CollationKeyFilter
converts each token into its binary CollationKey using the
provided Collator, and then encode the CollationKey
as a String using
IndexableBinaryStringTools, to allow it to be
stored as an index term.

Taxonomy of Categories
Facets are defined using a hierarchy of categories, known as a
Taxonomy
For example, in a book store application, a Taxonomy could have the
following hierarchy:
Author
Mark Twain
J.